Intermolecular structure around lithium monovalent cations in molten LiAlCl4

Abstract

The structure of molten LiAlCl₄ has been investigated by means of a time-of-flight neutron diffractometer, using the method of Li isotopic substitution. The distribution function G_Li(r) around Li⁺ was derived from the Fourier-transform of Δ_Li(Q), where it was given by the algebraic difference between the corrected neutron differential cross-sections of two melts, ⁷LiAlCl₄ and ^6,7LiAlCl₄. The latter includes no contribution from the Li nuclei in the observed differential cross-sections. The nearest-neighbour Li⋯Cl distance and the coordination number were found to be r(Li⋯Cl)= 2.72 Å and n_LiCl= 7.7, respectively, from the first peak in G_Li(r). The first peak in G(r) of molten ^6,7LiAlCl₄ showed that r(Cl⋯Cl)= 4.0 Å and n_ClCl= 8.4. The intramolecular geometry of the AlCl^–₄ ion was also determined from the diffraction data at 4 < Q/Å^–1< 15 for the ^6,7LiAlCl₄ melt. The intramolecular parameters r_AlCl= 2.180±0.008 Å and r_ClCl= 3.560±0.008 Å, indicated that AlCl^–₄ ions had a regular tetrahedral structure. The possible configuration among the AlCl^–₄ ions as well as the possible coordination by AlCl^–₄ of Li⁺ was illustrated.